Without limiting the scope of the present invention, its background is described in connection with performing tests to determine the production capabilities of a formation traversed by a wellbore, as an example.
During the course of drilling an oil or gas well, the wellbore is typically filled with a fluid known as drilling fluid or drilling mud. One of the purposes of this drilling fluid is to contain formation fluids within the formation intersected by the wellbore. To contain these formation fluids, the drilling mud is weighted with various additives so that the hydrostatic pressure of the drilling mud at the formation depth is sufficient to maintain the formation fluid within the formation without allowing it to escape into the wellbore.
When it is desired to test the production capabilities of the formation, a test string is lowered into the wellbore to the formation depth and the formation fluid is allowed to flow into the test string in a controlled testing program. Lower pressure is maintained in the interior of the test string as it is lowered into the wellbore. This is usually done by keeping a valve in the closed position near the lower end of the test string. When the testing depth is reached, a packer is set to seal the wellbore thus closing in the formation from the hydrostatic pressure of the drilling fluid in the well annulus. The valve at the lower end of the test string is then opened and the formation fluid, free from the restraining pressure of the drilling fluid, can flow into the interior of the test string.
The testing program typically includes periods of formation flow and periods when the formation is closed in. Pressure recordings are taken throughout the program for later analysis to determine the production capability of the formation. If desired, a sample of the formation fluid may be caught in a suitable sample chamber.
At the end of the testing program, a circulation valve in the test string is typically opened so that formation fluid in the test string may be circulated out. Since the hydrostatic pressure of the drilling fluid near the formation is generally much higher than the formation fluids in the test string, it is usually only necessary that the annulus be placed in fluid communication with the interior of the test string to start to reverse out the formation fluids from the test string. Following this circulation step, the packer may be released so that the test string may be withdrawn from the wellbore.
Typically, the circulating valves used in a test string may include a sliding sleeve that is opened in response to pressure in the annulus. It has been found, however, that when it is desirable to have more than one circulating valves in a test string to be operated at different times, each tool must be set to operate at a different pressure. Since 500 psi typically separates the pressures at which respective circulating valves will operate, extremely high pressures would be required to operate the later circulating valves in such a configuration, which may damage the well casing.
To overcome this problem, attempts have been made to utilize internal pressure operated circulating valves that operated in response to pressure in the test string. It has been found, however, that internal pressure operated circulation valves may be inadvertently opened as the result of an increase in the pressure within the test string. For example, when the test string is made up and lowered into the wellbore, it is desirable to periodically pressure test the test string to assure that the pipe joints have been adequately made up. Such testing requires closing of a valve in the lower part of the test string and applying pump pressure to the interior or the test string at the surface of the well. If the test string includes an interior pressure operated circulation valve, it may be inadvertently opened during such a test string pressure test.
It has also been found that internal pressure operated circulation valves may be inadvertently opened as the result of an unexpected increase in pressure from a formation that is not properly under control. If an internal pressure operated circulation valve is not operated during a testing program and is pulled out of the hole in the unoperated position, such a pressure upset from the formation could open an internal pressure operated circulation valve and allow formation fluids to be release at the surface.
Therefore, a need has arisen for an internal pressure operated circulation valve that will not inadvertently opened as the result of an increase in the pressure within the test string during a pressure test of the test string. A need has also arisen for such an internal pressure operated circulation valve that will not inadvertently open as a result of an unexpected pressure surge from the formation particularly when the internal pressure operated circulating valve is at or near the surface.